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Turning Diffraction on Its Head

As light waves squeeze between obstacles, they fan out, a phenomenon known as diffraction. Now, for the first time, physicists have instead used light to diffract matter in the form of electrons. The work, reported in the 13 September issue of Nature, underscores the complementary wavelike and particle-like nature of matter and offers a new method of probing fundamental physics.

Overlapping waves are the heart of diffraction. When a light beam passes through a diffraction grating, a series of finely spaced parallel slits, it casts a so-called diffraction pattern on the other side. Bright regions appear where the peaks of the light wave combine; where peaks and troughs cancel each other out, there's darkness. Physicists have tried and failed in the past to use light to diffract electrons. Now, one team reports success.

Physicist Herman Batelaan and his colleagues at the University of Nebraska, Lincoln, created a diffraction grating out of pure light. They split a laser beam in two, rerouted it, and recombined the two beams head-on. The overlapping laser beams produced a standing wave of light with "slits" just 200 nanometers wide. When an electron beam intersects the standing light wave at a right angle, it forms a distinctive "bright" and "dark" pattern of electron intensity beyond, heralding electron diffraction. The real trick in getting the experiment to work lies in creating an electron beam just 25 micrometers wide, or a quarter of the width of a human hair, says Batelaan. Electrons behave like waves, and light assumes the mantle of matter: "It highlights particle-wave duality, and that is one of the cornerstones of quantum mechanics," says Batelaan.

This is a "landmark experiment," says physicist Mark Raizen of the University of Texas, Austin, representing an important step in using light to manipulate electrons. The technique offers a way to split and recombine electron beams to create a device called an electron interferometer, which can probe atoms with about 10,000 times higher sensitivity than its optical counterpart. Electron interferometry could be used to investigate interactions between matter and charged particles and to study fundamental physics, Raizen notes.